Set the LAN IP and port to be mapped and the external network port in the auxiliary router, and write down the external network port number for future use.

Set the port mapping on the first-level router to the second-level route. Connect the external network port number of the secondary route just recorded and the IP set by the secondary route to the LAN port of the primary route, and set it to the primary route under the function of port mapping and virtual server.

That is to say, at this time, you can only specify the internal network service under the secondary route you want to map through the LAN IP of the main route and the external network port of the secondary route with the mapping port.

Set up and successfully enable DDNS service at the first layer of routing.

After all the configurations are successful, you can access the internal network service under the secondary route in the external network.

It should be possible to map the DMZ of the second-level route to the first-level, but only one IP intranet service can be mapped, which has never been tried.